In recent years, because of the increasing attention to energy issue, many new energy-efficient lighting tools are developed. Among them, the light-emitting diode (LED) has features such as high luminous efficiency, less power consumption, mercury-free and long life time, and becomes a very promising lighting tool for the next generation.
For the white light LED for lighting, there are many references discussing different producing methods. One method is to use the LED chip and phosphor powder. For example, blue light emitted from a blue LED chip is used to excite YAG (Yttrium Aluminum Garnet, Y3Al5O12) phosphor to emit yellow light, and a mixture of both the blue and yellow lights forms white light.
There are two common methods for phosphor coating, one is conformal coating method, and the other one is the remote phosphor method. As shown in FIG. 1, the conformal coating is to coat the phosphor 103 directly on each LED chip 102. Because the phosphor is coated directly on the LED chip 102, the thickness is much uniform. However, because the light from the phosphor is absorbed by the LED chip 102 and the carrier 101, the overall luminous efficiency is reduced. In addition, because the phosphor 103 is in direct contact with the LED chip 102, when the LED chip 102 operates to result in a high temperature environment of 100° C. to 150° C., the phosphor layer deteriorates gradually, and the luminous efficiency is affected.
The remote phosphor solves the problem of the conformal coating. FIG. 2 shows a light-emitting device of an LED chip with remote phosphor. The light-emitting device 20 comprises a carrier 201, an LED chip 202, a hemispheric package resin 204, and the phosphor layer 203 coated thereon. As shown in FIG. 2, as the phosphor layer 203 is separated from the LED chip 202, the problem that light from the phosphor layer 203 is absorbed directly by the LED chip 102 is avoided. And because the phosphor layer 203 is disposed away from the LED chip 202, it is more difficult for phosphor powders in the phosphor layer 203 to deteriorate due to the high temperature environment when the LED chip 202 operates.
However, the luminous efficiency is usually affected by the resin in the remote phosphor structure. FIG. 3A shows the propagating path of light after being emitted from the LED chip. According to Snell's law, as the refractive index (denoted by n) of the LED chip 302 is 2.4, and the refractive index n of the package resin 304 is 1.5, when light from the LED chip is incident to the surface of the package resin 304 with an angle less than the critical angle θc, like the condition shown by path A, the light is refracted and enters into the package resin 304. But when light from the LED chip is incident to the surface of the package resin 304 with an angle larger than the critical angle θc, like the condition shown by path B, the light is totally and internally reflected in the LED chip (Total Internal Reflection, TIR) and is absorbed by the LED chip 302. Therefore, when the refractive index difference between the LED chip and the package materials outside the LED chip is too large, the luminous efficiency of the LED chip is greatly affected.
In addition, there is the scattering effect of the particles of phosphors powder as shown in FIG. 3B. The phosphor powder particles 303a are excited to emit light of a different color by the light from the LED chip. However, the light emitted from the phosphor particles 303a propagates in all directions, and therefore, part of the light from phosphor powder particles 303a is incident toward the surface of the resin package 304. This results in inward-propagating light rather than outward-propagating light, and thus the luminous efficiency is reduced.